What Is Time

Briefly, time is the measurement of electron vibrations. We will discuss how electrons emit light and how atomic clocks use this property to measure time.

According to the theory of General Relativity, time is also relative. It can be affected by gravity and motion, and the rate at which it passes depends on who is watching it. Before discussing the reasons why this is so, a better understanding of the nature of time is necessary.

The Bohr and Quantum Mechanical model of the Hydrogen atom

All matter in made up molecules, molecules are made up of various combinations of atoms. Atoms are made up of a positively charged nucleus containing protons (positive) and neutrons (neutral), and is surrounded by negatively charged electrons. Further breakdown is unnecessary for the scope of this article.

According to the Bohr model of the single-electron Hydrogen atom, the electron moves around the nucleus in set orbits of specific energy levels. The number of orbits is always a whole number and is called the quantum number. Motion is limited to these paths only and an electron in an orbit closer to the nucleus has less energy than if it were in an orbit farther away. (p. 859, Physics)

The quantum mechanical model of the Hydrogen atom uses four different numbers to describe the movement of an electron. The quantum number, the orbital quantum number, the magnetic quantum number, and the spin number which respectively describe the number of potential levels, the angular momentum, the magnetic field, and the spin (similar to the way our planet spins on its axis) of an electron. Together they give a probability of where an electron is more likely to be. So rather than being in a specific orbit at a distinct radius from the nucleus, this model gives us a neighborhood of where an electron might be.

Atoms that contain more than one electron still use these four numbers to describe electron motion and energy. In this case, electrons with the same quantum number are said to be in the same “shell”, each shell containing a specific number of electrons depending upon the values of the remaining three values. (p.868, Physics)

Emission of photons as electrons change energy levels

To move from a lower to a higher level, an electron needs to pick up energy matching the difference between the two potential energy levels. This could happen when two atoms collide in a heated gas, for example. Conversely, when an electron gives the correct amount of energy to downshift into a lower orbit, it loses energy and emits a photon. (p.859, Physics)

A photon is an elementary particle that carries electromagnetic radiation. Depending on its frequency, this radiation can manifest as visible light or be invisible to the human eye such as microwaves. Photons, as all other elementary particles, also have the wave-particle duality behaving both as particles and as waves. (p.838, Physics) The wavelength of a wave is the length between two successive peaks, the period of a wave (T) is the time it takes for one cycle of a wave to move past a particular point, and the frequency is how fast it moves (f=1/T).

Caesium-133 and the atomic clock

Caesium is an atom with 55 protons in its nucleus or atomic mass of 55. The atomic number of an atom is the number of protons plus the number of neutrons in its nucleus. The nucleus of an atom can have any number of neutrons. When the number of neutrons does not equal the number of protons we have what is called an isotope. When a Caesium atom has atomic mass of 133 or 78 neutrons in its nucleus, we have what is called the isotope Caesium-133.

Individual photons may be guided to form an unbroken wave of light. This implies that all atoms must be “in the same state and ready to undergo the same internal rearrangement. And then, you must make each of them emit its particle of light at precisely the right moment …“ (p.57, Calder). According to Einstein, this can be accomplished by bringing another atom of the desired frequency close to the atom that needs to be “synched”; then the latter falls into step with the former much as a soldier would join a marching column. (p. 58, Calder)

Today’s atomic clock, the NIST-F1 clock, cools the atoms of cesium-133 to near absolute zero (-273 Celsius when atomic motion ceases) and registers the microwaves (waves of photons of a particular frequency) emitted by this atom as its electrons shift energy levels. The second as defined by the International System of Units, is the duration of 9,192,631,770 such cycles of radiation.(1) Caesium-133 is the only naturally occurring and only stable (not radioactive) isotope of cesium. (2) The duration of the second was chosen as such as specified by human measuring ability at the time it was adopted in 1967 (3).

So, philosophy aside, what is time? It is the vibration of electrons. Rather than saying “That was 9,192,631,770 vibrations of a Ceasium-133 isotope,” we say “That was one second.” This duration would show a different number of vibrations depending on the type of atom we were looking at. So as the atoms in our bodies, in the material things we see, in the universe, vibrate, “time” passes. And, according to Einstein’s theory of General Relativity, time depends on gravity, motion, and the observer.

Image source pixabay.com

Calder, Nigel EInstein's Universe. The Layperson's Guide, Penguin Books

Cutnell, John, & Johnson, Kenneth, Physics, 2nd. Edition, 1992, John Wiley & Sons

1. Wikipedia.org https://en.wikipedia.org/wiki/Caesium-133#Caesium-133

2. NIST.gov http://www.nist.gov/pml/general/time/atomic.cfm